Plastic parts may be economically manufactured through the thermoforming process. An extruded plastic sheet of a desired thickness is heated to a softening temperature and then brought into contact with a mold. A number of vacuum holes are drilled in the mold through which air is drawn to cause the heated and softened plastic sheet to conform to the shape of the mold. The formed part is cooled rapidly, by impinging air or liquid cooling of the mold, and separated from the mold for trimming and final treatment. In single sheet thermoforming the minor variations in sheet position or orientation with respect to the mold are of little importance, as once the sheet is in contact with the mold it takes on the desired shape.
Twin-sheet thermoforming presents tremendously increased possibilities for part formation, but brings with it significant challenges. In twin-sheet thermoforming two sheets are heated, and a first sheet is detached from a carrier and disposed on a lower mold, where a vacuum is drawn to conform the sheet to the lower mold. The second sheet is positioned over the first sheet and drawn upwardly into an upper mold, where partial vacuum is applied to cause the second sheet to conform to the upper mold. The two formed sheets are then brought together, still in the molds, and fused at those regions where the sheets touch. To expedite the heating, molding, and removal of the plastic sheets from the machine, the sheets are carried on a rotating carousel-type carrier, which is pivoted about a rotational bearing located at the center of the carousel. The carousel passes between the upper mold and the lower mold.
Because the upper and lower molds must leave room for the carousel to pass between, the two molds are mounted separately, and great care must be exercised to achieve the desired level of parallel orientation between the upper mold and the lower mold. In a common twin-sheet thermoformer, the lower mold is connected to a platen that is supported on a sinuous inflatable tube--typically similar to a fire hose. Pressure is applied to the lower mold by inflating the tube, thereby driving the softened plastic sheets on the two molds together. However, if the pressure is uneven, or if the mold itself is caused to flex by the applied pressures, there is bound to be variation in the degree of parallelness between the sheets. Any unevenness can result in undesirable variations in molded part thickness and sheet fusion. To achieve adequate part dimensions, much trial and error adjustment of the machine, shimming of the molds, and other time-consuming steps may be required, sometimes requiring many hours to prepare for production.
To provide an acceptable margin of error in part dimensions, manufacturers often must utilize starting sheet stock which is thicker than might be absolutely called for by the article design. All these labor-intensive adjustments and overspecification of plastic thickness add to the cost of the end product. In addition, the difficulty of obtaining close tolerances in the molded parts puts restrictions on the type and complexity of part which may be economically produced in the twin-sheet thermoforming process.
What is needed is a structure for supporting plastic sheets for twin-sheet thermoforming which will allow the sheets to be presented to highly parallel thermoforming molds in a repeatable operation.